Magnetic sensors utilizing the GMR effect, frequently referred to as xe2x80x9cspin-valvexe2x80x9d sensors, are known in the art. A spin-valve sensor is typically a sandwiched structure consisting of two ferromagnetic layers separated by a thin non-magnetic spacer layer. One of the ferromagnetic layers is called the xe2x80x9cpinned layerxe2x80x9d because it is magnetically pinned or oriented in a fixed direction by an adjacent antiferromagnetic layer, commonly referred to as the xe2x80x9cpinning layer,xe2x80x9d through exchange coupling. The other ferromagnetic layer is called the xe2x80x9cfreexe2x80x9d or xe2x80x9cunpinnedxe2x80x9d layer because the magnetization is allowed to rotate in response to the presence of external magnetic fields. In a spin-valve sensor, a change in resistance of a layered magnetic sensor is used to read data from a magnetic medium. This change is attributed to a spin-dependent transport of conduction electrons between the free magnetic layer and one or more pinned magnetic layers through the non-magnetic spacer layers.
Spin-valve sensors benefit from the change of resistance exhibited by the devices, which depends on the relative alignment between the magnetizations of the two ferromagnetic layers. In many practical spin-valve GMR heads, the layers have scattering at the boundaries that limits the size of the GMR. This occurs when the thickness of the layers is comparable with or smaller than the mean free path of the conduction electrons. A conduction electron remembers its spin memory through the sequential spin-dependent scattering in the free-layer, free-layer/spacer interface, spacer/pinned-layer interface and the pinned layer. The more interfaces the electron goes through without being scattered, the larger the GMR value. In the existing spin-valves applications, most of the electrons are scattered after entering the metallic capping layers or antiferromagnetic layer and no longer contribute to the GMR effect.
Specular reflections can be obtained by using insulators as capping layers and antiferromagnetic pinning layers. This enhancement to the GMR has been demonstrated in Co/Cu based spin-valves with NiO as the antiferromagnetic pinning layers. These spin-valves, however, may have disadvantages when used for GMR head applications due to their poor thermal stability.
In spin valve sensors, improved performance is partly measured by increased sensitivity, which is the ability of the sensor to detect magnetoresistive changes in a magnetic medium. As a result, it is desirable to find ways to improve the sensitivity of spin valve sensors. Consequently, spin valve sensors that respond strongly in the presence of electromagnetic fields are desired.
In general, in one aspect, the invention features a magnetoresistive sensor. The sensor includes a cap layer, a free layer, a spacer layer, a pinned layer, an oxide layer, a pinning layer, a seed layer, and a substrate layer. The sensor consists of the cap layer adjacent to the free layer. The free layer is adjacent to the spacer layer. The spacer layer is adjacent to the pinned layer. The pinned layer is adjacent to the oxide layer. The oxide layer is adjacent to the pinning layer. The pinning layer is adjacent to the seed layer, and the seed layer is adjacent to the substrate. Another aspect of the invention features a method of manufacturing the magnetoresistive sensor. This method includes forming a layered structure. The layered structure includes a cap layer, a free layer, a spacer layer, a pinned layer, an oxide layer, a pinning layer, a seed layer, and a substrate layer.
Implementations may include one or more of the following features. A second pinned layer can be inserted between the oxide layer and the pinning layer. A second free layer adjacent to a second oxide layer can be inserted between the cap layer and the free layer. A Ru layer adjacent to a third pinned layer can be inserted between the second pinned layer and the pinning layer. A second pinning layer, a second oxide layer, and a second pinned layer, each respectively adjacent, can be inserted between the cap layer and the free layer. A second pinning layer, a third pinned layer, a second Ru layer, a fourth pinned layer, a second oxide layer, a fifth pinned layer, and a second spacer layer, each respectively adjacent, can be inserted between the cap layer and the free layer. The free layer can include a bilayer, which is comprised of NiFe and either Co or Fe.
Implementations may include the following advantage. Theoretical calculations show that a 60-100% enhancement to the GMR can be achieved with such implementations of specular reflection.